1. Field of the Invention
The field of this invention is switching circuitry for electrical power applications, and more particularly to inverter circuits for driving electric motors, transformers and the like.
2. Description of the Prior Art
The invented circuit is intended for applications such as a drive system for motors such as shown in U.S. Pat. No. 3,780,324. The inverter shown in that patent, as shown in FIG. 1 therein, utilizes SCR(30) switching, commutating capacitors 29 and inductances 28, and reactance diodes 31. Such inverter circuits are also shown and described in the Standard Handbook for Electrical Engineers, Fink and Carroll, Tenth Edition, at section 12-37. One disadvantage to such circuits is that inverter circuit components with high voltage ratings were required, since these components were subject to the high voltage transients induced by the sudden current switching. Another disadvantage of such prior art inverter circuits is that they are subject to miscommutation, particularly at low motor speeds when the motor voltage is relatively low as is the energy available for commutation.
One technique which has been used to minimize miscommutation is to utilize a separate voltage source to precharge the commutating capacitors, thereby avoiding variation of available commutating energy with varying DC voltage. However, the voltage transients are still present, requiring components with high voltage ratings. R-C snubber circuits are typically used to bypass spurious harmonic frequencies and stabilize the SCR control circuitry. However, these resonant circuits tend to raise the voltage on the semiconductor components even more by "ringing up" the voltage.